In the packing, food, and pharmaceutical industries, there is an increasing need for suitable plastic materials to replace glass and metal packaging, especially for foods, beverages, and pharmaceutical products. The multilayer systems used in these sectors contain, as a rule, several layers of different polymers such as polyethylenes, polypropylenes, polyvinyl chlorides and fluorides, polyvinyl alcohols, polyethylene vinyl alcohols, polyamides, ionomeric polyesters, polycarbonates, polyacrylates, and other polymers, their blends and/or their mixtures.
These packing systems generally constitute protective, barrier, and supporting systems for the above-named products. In the widest sense they comprise packaging such as flexible tubes, pipes, bottles, containers, bags, cans, and the like.
However, for the above-mentioned applications, the plastic materials must have, in addition to good processability and good mechanical properties, a barrier layer which is highly impermeable or resistant to oxygen, carbon dioxide, water vapor, and other gases and gas mixtures, such as aromatics, perfumes, and other gases or hydrocarbons, both toxic and non toxic. Very often, materials which are good oxygen barriers are permeable to water vapor. Thus, polar polymers which contain hydroxyl groups, such as cellophane and polyvinyl alcohol, constitute good oxygen barriers, but they are extremely hygroscopic and lose their barrier properties to a significant extent when they are moist. Hydrophobic polymers (such as polyolefins) constitute good barriers to water vapor, but their resistance to oxygen is poor. Polymers which have a balanced ratio of hydrophobic and hydrophilic structures, as for example polyvinyl alcohols, ethylene vinyl alcohol copolymers, and nylon (e.g. nylon 6 and 6,6) show very good barrier layer properties when the water vapor pressure is low, but as soon as the latter rises, the barrier layer becomes oxygen-permeable, which is attributable to the plasticizing action of the absorbed water.
From U.S. Pat. No. 4,826,955, barrier layers of an amorphous nylon copolymer are known. This copolyamide is composed of a lactam, m-xylylene diamine, and terephthalic acid or isophthalic acid. A disadvantage of these copolyamides is that, in the moist state, the glass transition point (Tg) drops to very low values of about 50.degree. C.; as a result, cleaning of containers made therefrom with hot water at over 70.degree. C. for the purpose of reuse of the container is not possible.
U.S. Pat. No. 4,696,865 describes copolyamides of hexamethylene terephthalamide (6.T) and hexamethylene isophthalamide (6.I) for use in packing containers such as bottles, these copolyamide layers having very good barrier properties against oxygen and carbon dioxide. The weight ratio of terephthalic acid to isophthalic acid in the above copolyamide is between 1.0 and 1.9, more particularly between 1 and 1.5. These copolyamides have glass transition temperatures of about 135.degree.-165.degree. C.; however, this value drops in the conditioned (moisturized) state below 100.degree. C. Thus, repeated cleaning processes with hot water (having a temperature of over 60.degree.) cannot be carried out, so that reuse of such containers is not intended or is very limited.
For sanitary reasons it must be possible to clean bottles for sugar and dye containing beverages at at least 70.degree. C. As such prior art copolyamides are often already cloudy from their manufacture, they are not seriously considered for many types of packaging.
From EP-A-358038 are known nylon blends as well as single and multi-layer films and containers containing these nylon blends. These layers consist of an alloy of amorphous copolymers of hexamethylene isophthalamide/terephthalamide and a partially crystalline copolyamide, the latter having a melting point of at least about 145.degree. C. In this polymer combination, on the one hand the softening point of the amorphous copolyamide decreases due to the alloying effect of the partially crystalline copolyamide. On the other hand, in the conditioned state, the blend has a much lower strength and hot form stability than in the dry state. Such alloys also become cloudy and brittle after repeated contact with hot water.
In EP-A-378856, polycarbonate containers whose outer layers consist of polycarbonate and which have an inner barrier layer of a copolyamide of nylon 6.I/6.T (an amorphous polyamide of hexamethylene diamine and a phthalic acid composition which contains about 65 weight % isophthalic acid and 35 weight % terephthalic acid) are described. Such copolyamides have a glass transition point of about 125.degree. C. (dry) and less than 70.degree. C. (wet). In contact with hot water, these polymers become cloudy and susceptible to breakage, which also greatly limits their reusability.
U.S. Pat. No. 4,937,130 teaches multilayer polycarbonate containers which contain, between the inner aromatic polycarbonate and the outer aromatic polycarbonate layer (and additional adhesive layers of polycarbonate/polyamide blends), an intermediate layer of a similar amorphous polyamide which consists of hexamethylene diamine, isophthalic acid, terephthalic acid, and 1,4-bisaminomethylcyclohexane. It has a glass transition temperature in the dry state of about 105.degree. C. Upon repeated contact with hot water, however, this copolyamide becomes cloudy and susceptible to breakage, while the Tg falls below 60.degree. C. For sweetened beverages and fruit juices, containers made of these materials cannot be reused.
Polyamide compositions with improved gas barrier layer properties are known from EP-A-409666. These polyamides consist of hexamethylene and meta-xylylene isophthalamide, adipic acid, and possibly terephthalic acid units. The glass transition temperature (dry) of this copolyamide is only about 90.degree. C. Repeated cleaning of a container having such a polyamide barrier layer with hot water of over 70.degree. C. to permit reuse of the container is thus not possible.
The lactam-containing copolyamides mentioned in the foregoing patents have a further disadvantage in that monomers migrate out of the copolyamide layer; this is not acceptable in packaging for the food sector, or else is objectionable toxicologically. In addition, copolyamides which have lactam building blocks absorb larger amounts of moisture, whereby the quality of the foil is further impaired.
In the packaging sector, e.g. containers for carbonated and sweet beverages, there is a great need for copolyamides with high-grade and balanced barrier and mechanical properties, which inter alia will safely withstand repeated sterilizing with hot water at 70.degree. C. and higher, so that neither transparency, strength, flexibility, nor their barrier qualities are impaired.
While bottles for mineral water made of polyethylene terephthalate are being washed at 50.degree.-60.degree. C. and are reusable only to a limited extent, the cleaning requirements for containers for highly colored and sweetened beverages (with 15-20 cleaning cycles) are 80.degree. C. and higher during a cleaning cycle period of 10-20 min.